The present invention relates generally to industrial process field devices, and more particularly to DIN-type temperature transmitters.
The term “field device” covers a broad range of process management devices that measure and control parameters such as pressure, temperature, and flow rate. Many field devices include transmitters that act as communication relays between a transducer for sensing or actuating an industrial process variable, and a remote control or monitoring device such as a computer in a control room. The output signal of a sensor, for example, is generally insufficient to communicate effectively with a remote control or monitoring device. A transmitter bridges this gap by receiving communication from the sensor, converting this signal to a form more effective for longer distance communication (for example, a modulated 4-20 mA current loop signal, or a wireless protocol signal), and transmitting the converted signal to the remote control or monitoring device.
Process transmitters are used to monitor a variety of parameters of industrial processes, including pressure, temperature, viscosity, and flow rate. Each field device typically comprises a sealed enclosure containing a transmitter, one or more sensors, and connecting electronics. Process transmitters comprise wired or wireless transceivers, as well as electronics for processing sensor and control signals, diagnostic reports, and for receiving power. Large scale industrial manufacturing facilities typically employ many field devices distributed across a wide area. These field devices usually communicate with a common control or monitoring device, allowing industrial processes to be centrally monitored and controlled.
Sensor assemblies for sensing process fluid temperatures and changes in temperature commonly include at least one temperature sensor such as a thermocouple, resistive temperature detector, or thermistor housed in a thermowell extending into the fluid flow. The thermowell protects the temperature sensor from physical damage caused by direct contact with the process fluid (e.g. impacts, corrosion, etc.), while efficiently conducting heat between the fluid and the temperature sensor. DIN-type temperature sensors, in particular, comprise long, slender probes that extend from a flat mounting plate to a sensor tip containing a sensor element. The elongated probe situates the sensor tip at the bottom of the thermowell, surrounded by the process flow. Impingement of the thermowell on process flow creates turbulence in the process fluid via vortex shedding. This turbulence, in turn, contributes to vibration of both the thermowell and the sensor probe inside it. To avoid sensor damage caused by this vibration, many sensor assemblies mount the sensor flexibly with respect to process piping (e.g. via a spring loaded assembly), allowing the probe to move together with the vibrating thermowell. Flying leads connect the sensor probe to signal processing electronics within the process transmitter.
Field devices can be powered by direct electrical connection to power utilities such as 120V AC utilities, by control system DC power supplies, by energy storage devices such as long-life chemical batteries or supercapacitors, or by local power sources such as solar panels or vibrational energy harvesters. Field devices powered by local power sources or utility grids may experience transient power surges that can damage or degrade sensitive process transmitter electronics.